How will natural systems respond to changes in climate? This material is based upon work supported by the National Science Foundation under Grant Number DEB1257655

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the bad guys are moving up

Below are two separate discussions of a new article on “Crop pests and pathogens move polewards in a warming world” by Bebber et al. First, Brian Machovina provides a summary of the article and highlights the extreme importance of understanding the effects of climate change on food supply and food security. Second, Evan Rehm expands the discussion to think about how the new study fits within the context of a broader debate about “assisted migration” and the role of active management in minimizing the effects of climate change.

__________________________Poleward migration of pests by B. Macovina

The effects of climate change will perhaps be most profoundly felt by humans through one of the only things they cannot do without: food. Current agricultural zones are predicted to be affected by annual changes in temperature and precipitation, more frequent droughts or excessive rainfall events, and shifts in pests and pathogens. Although the spread of pests and pathogens is primarily due to human transportation, it has been believed that poleward increases of temperature could be accompanied by establishment of into regions previously free of specific crop pathogens. Between 10 and 16% of crops are estimated to be lost to pests, and any increase in pest distribution could increase losses.

Bebber et al. recently presented a study in the journal Nature Climate Change (“Crop pests and pathogens move polewards in a warming world”) indicating evidence of past geographic shifts in agricultural pathogens. They demonstrated an average poleward shift of 2.7±0.8 km per yr since 1960, in observations of hundreds of pests and pathogens, but with significant variation in trends among taxonomic groups. Examining the latitudes and dates of the earliest record of 612 crop pests and pathogens from two exhaustive historical databases, they determined whether the latitude of new observations has changed significantly over time. The study also looked for the presence of observational biases, caused by latitudinal gradients in the abilities of countries to detect, identify and report pests, and latitudinal trends in observations for individual pest species (i.e. higher GDP countries have more investigative resources).

Latitudinal trends in observations varied greatly among individual pest species, but significant positive latitudinal trends were detected when analyzing all species together. For Northern Hemisphere observations, the Acari, Bacteria, Coleoptera, Diptera, Fungi, Hemiptera, Isoptera, Lepidoptera and Oomycota show increased detection towards the north since 1960. In contrast, Nematoda and viruses show an opposite trend, shifting towards the Equator. Randomization tests showed that no trend should be detected, if no temporal pattern were present.

As temperatures are projected to rise, accompanied by shifts in rainfall, more geographic shifts in pests and pathogens are to be expected. Such changes, on top of a growing instability of climate, will likely increase the challenges faced by farmers of the future.

__________________________Hitching a ride: assisted migration may be more necessary than we think.by E. Rehm

A recent article published in the journal Nature Climate Change got me thinking about a topic that will likely become more prevalent as climate change intensifies, “Assisted migration”. This article, “Crop pests and pathogens move polewards in a warming world”, is focused on highly-modified human agricultural systems and made me think about how much human assisted migration is already occurring, whether intentional or accidental?

The debate about assisted migration is relatively new to ecology and conservation but in a nutshell it is based on the idea that humans can physically move individuals or entire populations to higher latitudes or altitudes to assist species persistence into the future. In an earlier post, Dr. Ken Feeley discussed how the majority of species worldwide are not keeping up with the pace of climate change through adaptation and thus will need to shift their distributions in order to avoid climate-driven extinctions. As we begin to realize that more and more species are migrating slower than they need to be, we will quickly see the value in assisted migrations.

Assisted migration does not come without controversy. Most opponents of the idea point to past examples such as the devastating effects of introducing the European rabbit to Australia. Purposefully moving species around the globe is not a new thing. What got my attention in this article was that for many agricultural species assisted migration is already occurring, whether we like it or not.

A quick look at the numbers shows that on average agricultural crop pests are shifting latitudinally at a rate of 2.7 kilometers per year (km/yr), which is much faster than the 1.7 km/yr shift found in naturally occurring bird, insect and mammal populations [1]. The authors specifically point to the agricultural trade system as one mechanism by which species are migrating. It is easy to see that humans are likely playing a large role in some pest migrations such as protozoa, which are slow moving, microscopic organism largely occurring in the soil yet they have migration rates of 60+ km/yr.

To me it is obvious that assisted migrations are already occurring and will likely become more regular as humans become more mobile. In this example we see how organisms that we perceive as negative, i.e. crop pests, are being shuttled about as humans move through the landscape. But what about the positives?

As a tropical biologists I view assisted migration as a necessary evil. The immense biological diversity held in the tropics is extremely valuable in my eyes and we are already seeing many species in peril because they cannot migrate fast enough. Given our lack of action and denial of ongoing climate change, assisted migration isn’t a question of ‘if’, but rather ‘when’ and ‘how’ do we get involved.

The “when” part of that question is easy, NOW. We immediately need to start planning and testing the feasibility of moving individuals, species or potentially entire ecosystems. The “how” is a lot more complicated. We have endless examples of how introducing a new species into an area has had severe environmental consequences. However, with today’s technology and a little bit of foresight we might be able to get things right this time around. As Dr. Camille Parmesan suggested in this 2009 Scientific American article we need to make informed decisions and have an ongoing open debate about the benefits and consequences of assisted migrations instead of continuing to ignore the issue.

Perhaps assisted migration can eventually be a positive way of saving some of our species or at least offsetting our past follies. For example, if we are already moving around agricultural pests resulting in decreased crop yields and increased chemical use, why can’t we move some of their natural predators as a way to potentially offset these negative effects? Assisted migrations is not the key to solving our problems but may be just one more tool in our ecological toolbox.